Cracking hydrocarbon feeds to obtain high yields of very good quality motor gasoline was begun in the petroleum industry at the end of the 1930s. The introduction of fluid bed processes (FCC, Fluid Catalytic Cracking) or moving bed processes (such as TCC) in which the catalysts continuously circulate between the reaction zone and the regenerator (where it is freed of coke by combustion in the presence of a gas containing oxygen), constituted a great advance over the fixed bed technique. Fluidised bed units (FCC) are now much more widespread than moving bed processes. Cracking is normally carried out at about 500.degree. C. at a total pressure which is close to atmospheric pressure and in the absence of hydrogen.
The prior art is illustrated, for example, in European patent EP-A-0 142 313.
Since the beginning of the 1960s, the most widely used catalysts in cracking units have been zeolites, normally with a faujasite structure. Such zeolites, incorporated in an amorphous matrix, for example constituted by amorphous silica-alumina, and which can contain clays in a variety of proportions, are characterized by hydrocarbon cracking activities which are 1000 to 10000 times higher than those of silica-alumina catalysts which are rich in silica which were used up to the end of the 1950s.
Near the end of the 1970s, the crude oil shortage and the increasing demand for high octane number gasoline led refiners to treat heavier and heavier crudes. Treating these latter constitutes a difficult problem for the refiner because of their high level of catalyst poisons, in particular metallic compounds (especially nickel and vanadium), unusual Conradson carbon numbers and, in particular, asphaltene compounds.
This need to treat heavy feeds and other more recent problems such as the gradual but general removal of lead based additives from gasoline, and the slow but substantial increase in demand for middle distillates (kerosines and gas oils) in some countries have also prompted refiners to research improved catalysts which can in particular satisfy the following aims:
catalysts which are thermally and hydrothermally more stable and more tolerant towards metals; PA1 which can produce less coke for an identical conversion; PA1 which can produce a gasoline with a higher octane number; PA1 which has improved selectivity for middle distillates. PA1 NU-86 zeolite has a three-dimensional microporous system; PA1 the three-dimensional microporous system is constituted by straight channels with a pore opening which is delimited by 11 T atoms (T being a tetrahedral atom principally selected from the group formed by Si, Al, Ga and Fe), straight channels which are alternately delimited by openings with 10 and with 12 T atoms, and sinusoidal channels which are also alternately delimited by openings with 10 and with 12 T atoms. PA1 a) 20% to 95% by weight, preferably 30% to 85%, and more preferably 50% to 80%, of at least one matrix; PA1 b) 1% to 60% by weight, preferably 4% to 50%, more preferably 10% to 40%, of at least one Y zeolite with faujasite structure; and PA1 c) 0.01% to 30% by weight, preferably 0.05% to 20%, more preferably 0.1% to 10%, of at least one NU-86 zeolite, which is dealuminated and at least partially in its acid form. PA1 contact time in the range 1 to 10000 milliseconds; PA1 catalyst to feed weight ratio (C/F) in the range 0.5 to 50; PA1 temperature in the range 400.degree. C. to 800.degree. C.; PA1 pressure in the range 0.5 to 10 bars (1 bar=0.1 MPa).
In the majority of cases, the production of light gases comprising compounds containing 1 to 4 carbon atoms per molecule is intended to be minimised and as a consequence, catalysts are designed to limit the production of such light gases.
However, in some particular cases demand for light hydrocarbons containing 2 to 4 carbon atoms per molecule, or some of them such as C.sub.3 and/or C.sub.4 hydrocarbons, more particularly propylene and butenes, has grown to a substantial level.
The production of a large quantity of butenes is particularly interesting when the refiner can use an alkylation unit, for example for C.sub.3 -C.sub.4 cuts containing olefins, to form an additional quantity of high octane number gasoline.
Thus the global high quality gasoline yield obtained from the starting hydrocarbon cuts is substantially increased.
The production of propylene is particularly desirable in some developing countries where there is a high demand for such a product.
The catalytic cracking process can satisfy this demand to a certain extent provided that, in particular, the catalyst is adapted to such a production. One effective method of adapting the catalyst consists of adding an active agent to catalytic masses, the active agent having the following two qualities:
1. it can crack heavy molecules with good hydrocarbon selectivity for 3 and/or 4 carbon atoms, in particular to propylene and butenes;
2. it must be sufficiently resistant to the severe steam partial pressure and temperature conditions which prevail in the regenerator of the industrial cracker.